The present invention relates generally to methods of controlling transmitting power in a cellular radio communication system. More particularly the invention relates to a fast, robust and adaptive power control method.
The invention also relates to an arrangement for carrying out the method.
The capacity of CDMA (Code Division Multiple Access) systems is interference limited since the channels are neither separated in frequency nor separated in time. A single user exceeding the limit on transmitted power could, in a surrounding area, inhibit the communication of all other users. Thus, power control is very important in interference limited systems such as CDMA or wideband-CDMA.
The power control systems have to compensate not only for signal quality variations due to a varying distance between base station and mobile terminal, but must also attempt to compensate for signal quality variations typical of a wireless channel. These variations are due to the changing propagation environment between the base station and the mobile terminal as the mobile terminal moves across the cell or as some elements in the cell move. There are basically two types of channel variations, slow fading and fast fading.
Several proposals have been made to mitigate these two types of fading in CDMA systems. A well-known prior art power control method is that which is found in interim standard 95 (IS-95) systems. The IS-95 reverse link power control mechanism consists of two parts, open loop power control and closed loop power control. The open loop power control is used to adjust the mobile terminals"" transmitting power based on the received power from the base station. Assuming that the radio environment is reciprocal in both forward and reverse link, the mobile terminal adjusts its reverse link transmission power according to the received power on the forward link, i.e. if the received power is for example 5 dB lower than expected, the mobile terminal raises its output power with 5 dB which is a good estimate of the path loss on the reverse link.
However, because of the frequency separation between reverse and forward links, the fast fading of the two links are independent. To account for this difference and to further control the mobile terminals"" transmit power, closed loop power control is used. In the closed loop power control mechanism, the base station demodulates the reverse link and determines the signal to noise ratio, SNR, of the user. If the SNR is lower than a desired threshold the base station orders the mobile terminal to raise the transmit power. If the SNR is higher than a desired threshold the base station orders the mobile terminal to lower the transmit power.
A significant part of the closed loop power control mechanism is the specification of a fixed power step size. Each power adjustment command orders the mobile terminal to either raise or lower its output power with 1 dB. The 1 dB fixed change per power order command is chosen based on a compromise of different radio environments, ranging from a stationary mobile terminal to a high speed vehicle and of different channel types.
If the signal quality changes quickly it is necessary for the power control mechanism to follow these variations, i.e. if the signal quality goes down a power command ordering the transmitter to raise the used output power should be issued. The 1 dB fixed change per power order command then causes problems, since, if the power step size is too large, i.e. the quantization is too small, unnecessary fluctuations occur around the desired power level causing unnecessary interference. If, on the other hand, the power step size is too small the power control mechanism is not capable of following fast variations in signal quality, since the required number of necessary power order commands grows too large, i.e. the slew rate is not large enough.
In WO-9726716 is described a method of dynamically controlling the power step size in such a way that, on the basis of the power order commands to be examined, a calculation is made of the number of two successive commands in different directions in proportion to the total number of commands during a specific time period. The calculated proportion is then compared to a reference value and the power step size is changed according to this comparison.
In WO-9851026 is described a method where the power step size is changed in discrete steps, e.g. 0.25, 0.50 and 1.00 dB based on system conditions, such as the speed of the mobile terminal.
The general problem with the currently known approaches is that the possibilities of adjusting the power step size are too coarse and that the methods are not robust enough against errors in the signalling channel, which can cause power drift between the actually used output power and the desired output power.
The present invention deals with the problem of how to achieve an adaptive power control method that is robust against errors in the signalling channel and at the same time reacts quickly to fast fading variations.
One way of realising a power control, which reacts quickly to fast fading variations, is to delta-modulate the desired power, that is, transmitting simple power commands on an in-band signalling channel that order the mobile terminal or the base station to either raise or lower its respective output power. In order to avoid large control loop delays these power commands should be transmitted without channel encoding and outside any interleaving. This implies that there will be bit errors in the power commands. The delta-modulation must therefore be robust to errors and at the same time able to react quickly to fading variations.
Thus, an object of the present invention is to realise a power control method in such a way that no strong fluctuations occur around the desired power level and that it is robust against errors and still capable of reacting quickly to, and following fast fading variations.
This object is achieved according to the invention by means of a fast power control method, in which method the desired power change is calculated by means of an adaptive algorithm, with channel quality and estimated previously used output power as input data. Power drift, caused by bit errors in the in-band signalling channel, between the desired output power and the actually used output power is reduced by multiplying the estimated previously used output power value with a xe2x80x9cforgetting functionxe2x80x9d, with values between 0 and 1, reducing the influence of the estimated previously used power outputs. Finally, at the receiving end the new output power from the transmitter is calculated.
In a first embodiment of the invention the average power step size is made dependent of the time correlation of channel quality. If the correlation of channel quality is high the power step size could be larger than if the correlation of channel quality is low, since the behaviour of the channel then is more uncertain.
In a second embodiment of the invention the xe2x80x9cforgetting functionxe2x80x9d xcex is made dependent on the time correlation of channel quality. If the correlation of channel quality is high the xe2x80x9cforgetting functionxe2x80x9d xcex should be large to give old values higher impact resulting in a faster system. If, on the other hand, the correlation of channel quality is low the xe2x80x9cforgetting functionxe2x80x9d xcex then should be made small to reduce the impact of old power values since the channels behaviour is uncertain.
In a third embodiment of the invention the power step size is dependent on the soft value of the power order command. If the soft value indicates a large probability for bit errors the power step size should be small and if the soft value indicates a low probability for bit errors the power step size should be larger since the power order then is more reliable.
In a fourth embodiment of the invention the xe2x80x9cforgetting functionxe2x80x9d xcex is made dependent on the soft value of the power command. If the soft value indicates a large probability for bit error the forgetting function xcex should be made small and if the soft value indicates a low probability for bit error the xe2x80x9cforgetting functionxe2x80x9d xcex should be made large.
In a fifth embodiment of the invention the power step size is made indirectly dependent on channel quality by defining an area within which a difference between the desired output power and a long term mean value of the desired output power is allowed to change.
By using the proposed power control method a fast power control is achieved with long-term robustness and good adaptability to the variations in the in-band signalling channel.
The term xe2x80x9ccomprises/comprisingxe2x80x9d when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.